Flexible Surface-Enhanced Raman Scattering Substrates: A Review on Constructions, Applications, and Challenges

Surface-enhanced Raman scattering (SERS) has great potential in the field of rapid detection of pesticide residues in food. In this paper, a fiber optic SERS sensor excited by evanescent waves was proposed for efficient detection of thiram. Silver nanocubes (Ag NCs) were prepared as SERS active substrates, which had much stronger electromagnetic field intensity than nanospheres under laser excitation due to much more "hot spots". By using the method of electrostatic adsorption and laser induction, Ag NCs were uniformly assembled at the fiber taper waist (FTW) for enhancing the Raman signal. Different from the traditional way of stimulation, evanescent wave excitation greatly increased the interaction area between the excitation and analyte, while reducing the damage of the excited light to the metal nanostructures. The methods proposed in this work have been successfully used to detect the pesticide residues of thiram and showed good detection performance. The detection limits for 4-Mercaptobenzoic acid (4-MBA) and thiram were determined to be 10-9 and 10-8 M, the corresponding enhancement factor could be 1.64 × 105 and 6.38 × 104. Low concentration of thiram was detected in the peels of tomatoes and cucumbers, indicating its feasibility in actual sample detection. The combination of evanescent waves and SERS provides a new direction for the application of SERS sensors, which had great application potential in the field of pesticide residue detection.

Fiber Optic SERS Sensor with Silver Nanocubes Attached Based on Evanescent Wave for Detecting Pesticide Residues.

Time domain optical coherence tomography is a useful tool for diagnosing otitis media with effusion

In this work, thermo-optic (TO) waveguide switches for 650 and 532 nm visible wavelengths are designed and fabricated by the metal-printing technique based on poly (methyl methacrylate—glycidyl methacrylate) [P(MMA-GMA)] material. The optical characteristics and thermal stability of the P(MMA-GMA) material are analyzed. Optical transmission modes in the core waveguide for different visible wavelengths are simulated, and the thermal field distribution from the self-heating electrode structure is calculated, respectively. The structural parameters of the devices compatible with 650 and 532 nm visible wavelengths are designed optimally. For 650 and 532 nm signal wavelengths, the insertion loss of the actual TO switch fabricated is less than 3.2 dB, and the response time of the device is about 367.4 μs at 100 Hz square wave electrical signals. The driving electrical power of the device for the 650 nm signal wavelength is 15.2 mW and 14.0 mW for the 532 nm signal wavelength, respectively. The extinction ratio of the visible TO switch for 650 nm is 15.1 dB and 18.5 dB for 532 nm, respectively. The technique is suitable for realizing plastic optical fiber system applications.

Metal-printing polymer waveguide thermo-optic switches compatible with 650 and 532 nm visible signal wavelengths for plastic optical fiber systems.

Different from ordinary planar waveguide structure, we designed a double-metal-cladding waveguide (DMCW) for easier light coupling into the guiding layer from free space. In contrast to evanescent waves in a surface plasmon polariton waveguide, an oscillating wave is generated in the guiding layer. and a similar Fabry–Perot (FP) cavity can be formed by the DMCW. In past work, the FP cavity excited by the DMCW was used to study the refractive index of light, while in this work, the FP cavity is used to excite the photothermal effect of the metal substrate. It is a good connection between light and heat. The photothermal effect is investigated to promote the galvanic replacement reaction in the substrate. Although the experiment process is destructive to the DMCW structure, a surface-enhanced Raman scattering (SERS) chip is prepared on the basis of the photothermal effect in the DMCW. It shows that the DMCW can convert the energy of incident light into thermal energy, and then prepare the SERS chip. The chip has better uniformity, stronger activity, and higher sensitivity. The results demonstrate that the morphology of the SERS substrate created via the DMCW is far more elaborate than that via the surface plasmon polariton waveguide.

SERS chip fabricated by the thermal effect in a double-metal-cladding waveguide

In order to double the mode density to increase the Raman scattering efficiency, a double cavity structure was introduced to the metal-cladding optical waveguide (MCW) chip. This novel double cavity MCW inherits all the merits of the MCW, such as free space coupling, large detection area, and polarization independence. Furthermore, attributed to the doubled mode density, more incident energy is coupled into the guiding layer and results in a larger enhancement of the Raman scattering. Since the distribution of the electric field is sensitive to the refractive index profile of the two cavities, the proposed structure can be further optimized to concentrate all of the energy to the sample cavity. Experimental verification via four liquid samples of a different refractive index is carried out in support of the above hypothesis.

Enhanced Raman spectroscopy by a double cavity metal-cladding waveguide

We report experimental realization of Raman spectra enhancement of copper phthalocyanine, using an on-chip metallic planar waveguide of the sub-millimeter scale. The oscillating ultrahigh order modes excited by the direct coupling method yield high optical intensity at resonance, which is different from the conventional strategy to create localized “hot spots.” The observed excitation efficiency of the Raman signal is significantly enhanced, owing to the high Q factor of the resonant cavity. Furthermore, effective modulation of the Raman intensity is available by adjusting the polymethyl methacrylate (PMMA) thickness in the guiding layer, i.e., by tuning the light–matter interaction length. A large modulation depth is verified through the fact that 10 times variation in the enhancement factor is observed in the experiment as the PMMA thickness varies from 7 to 23 μm.

/pdf/simple-raman-scattering-sensor-integrated-with-a-metallic-3llaq2uosu.pdf

Simple Raman scattering sensor integrated with a metallic planar optical waveguide: effective modulation via minor structural adjustment

With regard to surface-enhanced Raman spectroscopy (SERS), the preparation of substrates with high homogeneity and low cost remains a challenge. In this paper, cheap commercial DVD-R plates were adopted as supports, whose 3D periodic structure was transferred onto the surface of flexible polydimethylsiloxane (PDMS) easily. Then, silver nanoparticles were grown both on DVD and PDMS substrates by the in situ reduction method, and the SERS performances of these two substrates were investigated. The results confirmed that the PDMS-based substrate exhibited better enhancement performance and higher uniformity (RSD=4.16%). In addition, due to the flexibility and transparency of PDMS, it is not restricted by the surface shape of the object when applied in in situ detection. This low-cost, simple method will be widely used in the in situ detection of surfaces of objects of any shape.

In situ synthesis of silver nanoparticles on periodic supports as highly active and flexible surface-enhanced Raman spectroscopy substrates.

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Papers

SERS chip fabricated by the thermal effect in a double-metal-cladding waveguide

Enhanced Raman spectroscopy by a double cavity metal-cladding waveguide

Simple Raman scattering sensor integrated with a metallic planar optical waveguide: effective modulation via minor structural adjustment

In situ synthesis of silver nanoparticles on periodic supports as highly active and flexible surface-enhanced Raman spectroscopy substrates.